As part of a program to improve air quality standards, attempts have been made to control vapor emissions at gasoline service stations. Gasoline vapors commonly consist of photochemically reactive hydrocarbons. They react with oxides of nitrogen such as nitrous-oxide in the presence of sunlight to create smog. Tests indicate that approximately 4 grams of vapor are emitted for every gallon of gasoline deposited in a vehicle gasoline tank. Proportional amounts of vapor are released when the underground storage tank is refilled.
During the refueling of an automobile fuel tank, the vapors in the fuel tank are displaced as gasoline fills the tank volume. These vapors may be drawn from the fuel tank and delivered to the storage tank from which the gasoline is pumped. A suction blower has been utilized to accomplish this function.
Leaks, vents, and loose fittings have allowed excess air or vapors to be drawn into the storage tank. These cause a larger than necessary vapor volume to be transferred to the storage tank. The excess vapors must be released to prevent excessive pressurization of the storage tank.
Prior efforts to dissipate the excess pressure in the storage tanks have met with only limited success. The simplest process is known as the "balanced" system. It consists of a special nozzle and an ordinary underground piping. The automobile tank and the underground storage tank exchange vapor volume for liquid volume. Excess vapors generated by temperature variations, liquid traps, spit-back from vehicle tanks, pipe restrictions, and poor fits at the vehicle tank nozzle interface cause the balanced system to be very inefficient.
Attempts have been made to improve the efficiency of the "balanced" system by developing various "secondary" or "vacuum assist" systems. The "secondary" systems utilize a small vacuum pump to draw the vapors from the vehicle tank spout. The vapors are then processsed by refrigeration/condensation, catalytic oxidation, or incineration. Adsorption of the vapors on activated carbon beds is another method that has been tried. The carbon beds are regenerated by the reverse flow of air. These systems experience many serious deficiencies. Refrigeration/condensation is only advantageous for bulk storage facilities or stations pumping on the order of 100,000 gallons per month. Furthermore, they require cumbersome and costly refrigeration equipment for condensation. These units are also wasteful from a standpoint of energy conservation.
Yet another system that has been tried operates on the principle of converting hydrocarbon vapors to carbon dioxide and water vapor. These systems utilize a platinum or other noble metal catalyst for oxidation. The control of reactive temperatures is critical. Above 1200.degree. F, the life of the catalyst is greatly reduced. Below 900.degree. F, the conversion efficiency drops rapidly. To minimize the size of the reactor system, carbon absorption beds are used for intermediate storage of vapors. The activated carbon beds smooth out the large flow during vehicle refueling and permit slow regeneration over a period of time. However, the carbon beds are not completely effective. The lighter hydrocarbons fractions, such as methane or ethane are not readily absorbed and pass through the carbon beds without being captured. The heavier fractions, such as hexane or heptane are readily absorbed but are also difficult to desorb. They tend to remain after bed stripping and decrease the ability of the carbon bed to absorb subsequent vapors.
Another approach utilizes incineration to burn off the vapors. This method is also plagued with many problems. The flow of vapors is variable depending upon the number of vehicles refueled in a given period of time. The concentrations of hydrocarbons in the vapors varies greatly. Specifcially, the concentration is sometimes insufficient to support combustion and excess air causes the flame to extinguish. Also, a concentration that is too high will cause incomplete combustion with the by products thereof being released to the atmosphere. Several incineration systems have attempted, with little success, to solve these problems. One system utilizes carbon beds as a storage medium. Desorption of the hydrocarbons is made at a specified rate to control the combustion process. In this system, the inherent problems of utilizing carbon beds is similar to those previously described. Furthermore, the system wastes energy since the blower must be used during the desorption cycle. Another system burns the vapors in a continuous furnace. This system requires additional fuel to maintain the furnace flame. Other systems use air ejectors to scavenge vapors from the underground tank. The fixed flow ejector cycles on and off to maintain a fixed vacuum in the underground storage tank. This system causes unnecessary boil-off of gasoline.
Still other problems associated with any and all of the aforementioned systems concern bulk deliveries made to service stations. In the fall, it is common for gasoline manufacturers to switch to higher volatility gasoline. This improves low temperature performance in automobiles. When this higher volatility gasoline is dropped on top of older gasoline, excess vapors are created. The prior art systems are not capable of processing this large volume of excess vapor.
Therefore, there has been a need for the development of a system that safely and efficiently collects and disposes of hydrocarbon vapors. Some of the characteristics of an efficiently operated system should include the elimination of auxiliary blowers and carbon canisters, the prevention of cracks or leaks in the system by maintaining low pressure in the piping and the storage tank, the utilization of excess pressure in the tank as the driving force for the discharge of vapors therefrom, the burning of vapors in ambient atmosphere, effective monitoring of the system and interrupting the system in the event of operating abnormalities.